1. Field of the Invention
The present invention relates to a misfiring detection apparatus for an internal combustion engine which detects a misfire in a cylinder of the internal combustion engine by measuring ion current flowing between the center and ground electrodes of a spark plug attached to the cylinder.
2. Description of the Related Art
A known technique for detecting misfiring or the like of an internal combustion engine utilizes ion current. Ion current flows, after spark discharge of a spark plug attached to the internal combustion engine, due to ions present in the vicinity of the electrodes of the spark plug.
An example of such a technique is disclosed in Japanese Patent Application Laid-Open (kokai) No. 4-54283 (xe2x80x9cApparatus for Detecting Misfiring of Internal Combustion Enginexe2x80x9d). In this technique, an ion current component is integrated by means of a current-component detector (integrator); and a value obtained by integration is compared with a predetermined threshold value in order to detect misfiring of the internal combustion engine.
3. Problems Solved by the Invention
However, in the above misfiring detection apparatus for an internal combustion engine, since the ion current component is integrated by means of a current-component detector (integrator), as shown in FIG. 6, not only the ion current component but also an unnecessary discharge noise component is integrated.
Since the absolute level of the discharge noise component, which stems from residual energy of an ignition coil and is superposed on the ion current, tends to be higher at the time of misfiring as compared with at the time of firing, the ion current component includes many discharge noise components during misfiring. Further, the peak value of the discharge noise component is far larger than the ion current component, although the duration of the discharge noise component is shorter than that of the ion current component.
Therefore, the discharge noise component accounts for a non-negligible portion of the integration value output from the current component detector. Thus, such discharge noise component is a source of error in the integration value output from the current component detector, which error may reduce the accuracy in detecting misfiring.
Moreover, since the peak value of such discharge noise component varies depending on the state of combustion within a relevant cylinder, a discharge noise component having a peak value greater than the input limit voltage of the current component detector may be input to the current component detector. This may cause a failure of the current component detector or other components.
The present invention has been achieved in light of the above-described problems of the prior art. It is therefore an object of the present invention to provide a misfiring detection apparatus for an internal combustion engine which can improve accuracy of misfiring detection.
The present invention achieves the above object in a first embodiment by providing a misfiring detection apparatus for an internal combustion engine which detects a misfire in a cylinder of the internal combustion engine by measuring ion current flowing between the center and ground electrodes of a spark plug attached to the cylinder. The apparatus is characterized as comprising:
accumulating means for accumulating periods of time during which the ion current exceeds a predetermined current value; and
misfiring detection means for judging that misfiring has occurred when a total of periods accumulated by the accumulating means is not greater than a predetermined value.
In a second embodiment, the misfiring detection apparatus for an internal combustion engine is characterized in that the accumulating means comprises:
comparison means for (i) judging whether the ion current is not less than the predetermined current value by comparing the ion current to the predetermined current value, and (ii) outputting a comparison result; and
counting means for counting periods of time during which the comparison result output by the comparison means indicates that the ion current is not less than the predetermined current value.
In a third embodiment, the detection apparatus for an internal combustion engine of the second embodiment is characterized in that the counting means includes a CR integration circuit.
In a fourth embodiment, the misfiring detection apparatus for an internal combustion engine of the second embodiment is characterized in that the counting means includes a digital counting circuit.
In the first embodiment, the accumulating means accumulates periods of time during which the ion current flowing between the center and ground electrodes of a spark plug exceeds a predetermined current value; and the misfiring detection means judges that misfiring has occurred when a total of periods accumulated by the accumulating means is not greater than a predetermined value. Thus, the ion current can be determined as a cumulative value of ion current generation periods. That is, since the ion current is determined not as a cumulative value of current-related values but as a cumulative value of time-related values, even when the ion current contains a discharge noise component, the discharge noise component having a short duration accounts for only a small portion of the cumulative value.
In the second embodiment the accumulating means includes comparison means and counting means. The comparison means judges whether the ion current is not less than the predetermined current value by comparing the ion current to the predetermined current value and outputs a comparison result. The counting means counts periods of time during which the comparison result output by the comparison means indicates that the ion current is not less than the predetermined current value. Thus, the function of accumulating periods of time during which the ion current exceeds a predetermined current value can be realized by the comparison means and the counting means.
In the third embodiment, since the counting means includes a CR integration circuit, the counting means can be realized by an analog circuit. Thus, the cumulative period of time during which the ion current exceeds a predetermined current value can be detected as an analog value.
In the fourth embodiment, since the counting means includes a digital counting circuit, the counting means can be realized by a digital circuit. Thus, the cumulative period of time during which the ion current exceeds a predetermined current value can be detected as a digital value.